The beat of the heart is controlled by the sinoatrial node, a group of conductive cells located in the right atrium near the entrance of the superior vena cava. The depolarization signal generated by the sinoatrial node activates the atrioventricular node. The atrioventricular node briefly delays the propagation of the depolarization signal, allowing the atria to drain, before passing the depolarization signal to the ventricles of the heart. The coordinated contraction of both ventricles drives the flow of blood through the body of a patient. In certain circumstances, the conduction of the depolarization signal from the atrioventricular node to the left and right ventricles may be interrupted or slowed. This may result in a dyssynchrony in the contraction of the left and right ventricles, which may lead to heart failure or death.
Cardiac resynchronization therapy (CRT) may correct the symptoms of electrical dyssynchrony by providing pacing therapy through medical electrical leads to one or both ventricles or atria to encourage earlier activation of the left or right ventricles. By pacing the contraction of the ventricles, the ventricles may be controlled so that the ventricles contract in synchrony. One form of CRT is fusion pacing. Fusion pacing typically involves left ventricle (LV) only pacing with an electrode on the LV medical electrical lead in coordination with the intrinsic right ventricle (RV) activation. Effective fusion requires, for example, that the timing of the LV pacing be in synchrony with the earliest activation on the RV chamber. Fusion pacing can also involve pacing the RV with an electrode on the RV medical electrical lead in coordination with the intrinsic LV activation; however, RV only pacing is avoided because it can be arrhythmogenic in some patients and LV heart failure is more common than RV heart failure.
Achieving a positive clinical benefit from CRT is dependent on several therapy control parameters including the relative timing of pacing pulses delivered to effectively capture the right or left ventricles. Presently, CRT algorithms rely on a pre-excitation interval (e.g. 50-60 milliseconds (ms)). Pre-excitation interval is the time-interval that occurs before RV sensing in which pacing pulses are delivered to the LV. Conventional CRT algorithms do not take into consideration the fact that physicians may not consistently place the RV lead in the same or similar location for each patient. Consequently, in some cases, the RV sense time may significantly differ from the time of the onset of activation or the time of the earliest activation. For example, if the RV lead is in an electrically late area (e.g. RVOT), the time of the onset of activation occurs late (e.g. 70-80 ms after onset of depolarization). The timing of the delivery of LV pacing is calculated as 70-80 ms minus 50-60 ms which means the pacing stimulus can be delivered at 20-30 ms after onset of depolarization. The QRS complex on an electrocardiogram or an electrogram represents a summation of the advancing depolarization wavefronts through the ventricles. Pacing into the QRS complex or after onset of the QRS complex is not ideal for effective capture and does not provide the patient with full benefit of CRT. It is therefore desirable to develop additional methods or systems that are able to address the limitations associated with conventional CRT algorithms.